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The Mexican parliament intended to establish a commonwealth under the Spanish king, Ferdinand VII. King Ferdinand, however, refused to recognize Mexico's independence and said that Spain would not allow any other European prince to take the throne of Mexico. Instead, by request of Parliament, the prominent general and president of the regency Agustín de Iturbide was proclaimed emperor of Mexico.

The First Mexican Empire was short-lived, lasting less than two years, and was ruled by only one emperor, Agustín I of Mexico, who reigned for less than eight months. A republican rebellion based on the Plan of Casa Mata then overthrew the First Empire, establishing a Federal Republic of the United Mexican States in 1824.

The various independentist factions in revolutionary Mexico coalesced around three principles, or "guarantees," for Mexican independence from Spain: that Mexico would be an independent constitutional monarchy governed by a conservative European prince; that criollos and peninsulares would henceforth enjoy equal rights and privileges; and that the Roman Catholic Church would retain its privileges and position as the official religion of the land. These Three Guarantees formed the core of the Plan of Iguala, the revolutionary blueprint which, by combining the goal of independence and a constitution with the preservation of Catholic monarchy, brought together all factions.[2]

After officially declaring independence on 28 September 1821, the Mexican parliament intended to establish a commonwealth whereby the King of Spain, Ferdinand VII, would also be emperor of Mexico, while both countries would be governed by separate laws and form separate legislative bodies. Should the king refuse the position, the law provided for another member of the House of Bourbon to accede to the Mexican throne. Ferdinand VII, however, refused to recognize Mexico's independence and said that Spain would not allow any other European prince to take the throne of Mexico.

General Agustín de Iturbide, a Mexican criollo who led the insurgents in the final phases of the war, was elected head of the provisional government and of the regency which held the imperial power while a monarch was chosen. Iturbide was extremely popular after his successes in the war of independence, and on the night of 18 May 1822 a mass demonstration led by the Regiment of Celaya, which Iturbide had commanded during the war, marched through the streets and demanded that their commander-in-chief accept the throne himself.

On 19 May 1822, the Sovereign Congress named Iturbide emperor. On 21 May it issued a decree confirming this appointment, which was officially a temporary measure until a European monarch could be found to rule Mexico. Iturbide's official title was, "By Divine Providence and the National Congress, First Constitutional Emperor of Mexico" (Spanish: Por la Divina Providencia y por el Congreso de la Nación, Primer Emperador Constitucional de México). His coronation took place on 21 July 1822 in Mexico City.

Several insurrections arose in various Mexican provinces beginning in December 1822, but they were all put down by the Imperial Army except for Santa Anna's forces in Veracruz. Santa Anna had secretly persuaded General Echávarri, the commander of the Imperial forces, to switch sides and support the revolution when it was ready to be proclaimed throughout Mexico.

In December 1822, Generals Santa Anna and Guadalupe Victoria wrote and signed the Plan of Casa Mata, an agreement between these two generals, amongst other Mexican generals, governors, and high-ranking governmental officials, to abolish the monarchy and replace it with a republic. This plan did not recognize the First Mexican Empire and called for the convening of a new Constituent Congress. The insurrectionists sent their proposal to the provincial governments and requested their adherence to the plan. In the course of just six weeks, the Plan of Casa Mata travelled to such remote places as Texas, and almost all the provinces supported the plan.

By accepting the plan, a provincial government withdrew its allegiance from the Imperial government and assumed sovereignty within its province. By the agreement with Echávarri, the Plan of Casa Mata was proclaimed throughout Mexico on 1 February 1823. Emperor Agustín was left isolated with little support outside of Mexico City and a few factions of the Imperial Army. Consequently, he re-installed the Congress, which he had previously abolished, abdicated the throne, and fled the country on 19 March 1823. Santa Anna and the other proponents of the Plan of Casa Mata went on to oversee the drafting of a new constitution and the establishment of the First Mexican Republic the following year.

However, after the emperor abdicated, the Central American provinces seceded to form the Federal Republic of Central America, with only the province of Chiapas choosing to remain a part of Mexico. Subsequent territorial losses over the next several decades (principally to the United States) would eventually reduce Mexico to less than half its maximum extent.